Micromirrors and micromirror array devices are a type of microelectromechanical devices. A typical micromirror generally has a reflective and deflectable mirror plate that is attached to a hinge such that the mirror plate can rotate along a rotation axis. The rotation of the mirror plate can be achieved through an application of an electrostatic force derived from an electrostatic field that can be established between an addressing electrode and the mirror plate.
It is generally advantageous to drive the mirror plate with as large a voltage as possible. For example, a large actuation voltage increases the available electrostatic force available to move the mirror plate. Greater electrostatic forces provide more operating margin for the micromirrors—increasing yield. Moreover, the electrostatic forces actuate the mirror plate more reliably and robustly over variations in processing and environment. Greater electrostatic forces also allow the hinges of the micromirrors to be made correspondingly stiffer; stiffer hinges may be advantageous since the material films used to fabricate them may be made thicker and therefore less sensitive to process variability, improving yield. Stiffer hinges may also have larger restoration forces to overcome stiction. The switching speed of the mirror plate from an angle to another may also be improved by raising the drive voltage.
The application of a high-voltage, however, is often limited due to many facts, such as the limitation of the addressing electrode. In current micromirrors, establishment of the electrostatic field used for driving the mirror plate is accomplished by applying a voltage to the addressing electrode, and the voltage on the addressing electrode is determined by an output voltage of a circuit, such as a memory cell fabricated on a semiconductor wafer. Due to the limited output voltages of the circuitry on the semiconductor wafer, the voltage applied to the addressing electrode is constrained within a certain range. As a result, the electrostatic force derived from the electrostatic field that results from the voltage on the addressing electrode is limited within a certain range. In addition, a high voltage can be disadvantageous to the micromirror device. For example, a high voltage may cause unfavorable charging accumulation on the micromirrors.
Therefore, what is needed is a micromirror device wherein the electrostatic force is maximized for a given voltage on the addressing electrode of the micromirror.